fix(scene): unfreeze open on huge xref Inserts

A real DWG with an xref containing ~74k sub-entities froze on the first
render: Insert tessellation exploded the block, style-resolved every sub
serially, and one bad sub could hang the loop forever.

- Insert short-circuits to an insertion marker when the referenced block
  has more than INSERT_SUB_LIMIT (5_000) sub-entities. Documented as
  temporary in RENDERING_OPTIMIZATION.md.
- Re-run purge_corrupt_entities after resolve_xrefs so parser-garbage
  inside xref content is caught (initial purge runs before xref load).
- build_derived_caches: single pass over entities collecting handles
  per kind; tessellation reads via doc.get_entity instead of cloning
  every EntityType. Image decode now parallel too.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/update.rs

@@ -132,6 +132,17 @@ impl H7CAD {
                 if let Some(base_dir) = path.parent() {
                     let xrefs =
                         crate::io::xref::resolve_xrefs(&mut self.tabs[i].scene.document, base_dir);
+                    // xref content arrives un-purged: parser-garbage entities
+                    // inside the referenced file can trigger infinite loops in
+                    // tessellation. Run the corrupt-entity guard again.
+                    let extra_dropped = crate::io::purge_corrupt_entities(
+                        &mut self.tabs[i].scene.document,
+                    );
+                    if extra_dropped > 0 {
+                        self.command_line.push_error(&format!(
+                            "Warning: {extra_dropped} corrupt xref entities dropped"
+                        ));
+                    }
                     for info in &xrefs {
                         match info.status {
                             crate::io::xref::XrefStatus::Loaded => {